Experiments are planned to expand upon several areas of study of the regulation of prolactin (PRL), luteinizing hormone (LH), growth hormone (GH) and thyrotropin (TSH) secretion. Studies will be conducted to determine the effect of chronic treatment with synthetic thyrotropin-releasing hormone on plasma levels of PRL, TSH and GH, and on nitrogen retention and rate of weight gain in lambs. In addition, the influence of synthetic "somatostatin" (growth hormone release inhibiting factor) on the GH and PRL responses to arginine, and the PRL and TSH responses to TRH will be studied. Studies will also be conducted to examine the influence of apomorphine stimulation of dopaminergic neural transmission and dihydroxyphenylserine (DOPS) stimulation of norepinephrine on the secretion of PRL, LH, GH and TSH. Experiments are planned to examine, in more detail, the influence of ovarian steroid hormones and of amino acids on the metabolic clearance rates and actual secretion rates of PRL. The occurrence of the nocturnal increase in the secretion of PRL and GH will be studied in immature and mature male and female sheep.